The inventive concept relates to an image sensor, and more particularly, to a method of detecting defects in an image sensor which may occur in a floating diffusion area of the image sensor.
Unit pixels constituting a complementary metal oxide semiconductor (CMOS) image sensor comprise a photo diode generating image charges corresponding to an image signal, a transmission transistor transmitting the image charges to a floating diffusion area, a reset transistor resetting the floating diffusion area, a conversion transistor generating a voltage signal corresponding to the image charges accumulated in the floating diffusion area, and a selection transistor outputting the voltage signal generated by the conversion transistor.
When an image signal is applied to an N-type diffusion area of a photo diode, image charges, that is, electron-hole pairs, are generated in the N-type diffusion area of the photo diode by a photoelectric effect. The generated electrons are transmitted to a floating diffusion area via a transmission transistor, and the generated holes are discharged to a ground voltage connected to a P-type diffusion area of the photo diode.
Accordingly, a width of a dynamic range of an image sensor is determined by whether all the electrons generated in the photo diode can be transmitted to the floating diffusion area, regardless of the number of the electrons. A method for increasing the width of the dynamic range is generally accomplished by a method of increasing an area of the photo diode and decreasing the width and thickness of a plurality of metal lines that are means for electrical connections.
The method of decreasing the width and thickness of the metal lines may cause defects in the image sensor due to an electrical characteristic of materials for forming the metal line. The materials for forming the metal line may be copper, aluminum, or the like. When the metal line is formed of the above materials to be slim and narrow, uniformity of the metal line may be not achieved. Also, as spacing between the metal lines decreases, an actual short caused by migration of charges between adjacent metal lines may occur.
An image sensor includes a plurality of unit pixels arranged in a two-dimensional array. When the defects described above occur in a unit pixel from among the plurality of unit pixels, the unit pixel is brighter or darker than unit pixels not having a defect as described above. The defects described above may also be represented as a vertical or horizontal line.
Since the unit pixels having the defects described above operate normally, a general examination with respect to an image sensor cannot detect the defects described above. Also, the defects described above cannot be easily distinguished from defects that occur on a photo diode constituting an image sensor when foreign materials, such as dust, or the like, cut off an image signal applied to the photo diode.
The above defects, which may occur due to leakage current between a floating diffusion area and a substrate, can be detected, but when performing a reliability examination on a completed product, manufacturing costs of image sensors are increased.